The measurement of natural gamma rays (NGR) in oil well logging dates back to the late 1930s and is the oldest known nuclear well log measurement. In the early days of NGR logging, the NGR measurement was typically performed using Geiger-Müller (GM) tubes. While these tubes are robust and work at high temperatures, they exhibit poor sensitivity to gamma-rays and, thus, today, they are rarely used in logging. The GM tube was replaced in the 1960s and 70s by scintillation detectors comprising a scintillation crystal, which is typically NaI, coupled to a photomultiplier tube (PMT). NaI detectors show a large increase in gamma-ray sensitivity compared to GM-tubes and allow spectral measurements.
Over the last decade, there has been a renewed push to make downhole logging tools work at higher temperatures, for example above 175° C. Such high temperatures pose issues for many sensors and their associated electronics currently used by downhole logging tools. For example, the scintillators mentioned above have drawbacks at high temperatures. For a NaI scintillation detector, the light output, i.e. the number of scintillation photons per unit of deposited gamma-ray energy, drops as the temperature increases. At the same time, the quantum efficiency of the PMT's photocathode, i.e. the fractional number of electrons emitted per photon entering the photomultiplier, drops. Additionally, the noise (dark current) of the PMT increases. This makes it increasingly difficult to distinguish noise from real events as the temperature goes up.
Despite these drawbacks with scintillation detectors, their sensitivity makes them useful. As such, new methods of addressing these drawbacks are desirable.